Hook and loop fasteners, such as those currently marketed under the trademark Scotchmate.TM. by 3M, are a common mechanical fastener. One common form of hook is a mushroom-shaped hook which can also be used as a hermaphroditic mechanical fastener by engaging other hooks rather than loops. These hook structures, formed on webs to create a fastener, are one common type of stemmed web. A stem means a protrusion from a surface, such as a web, regardless of its shape, length, length-to-width ratio, geometry or other characteristics.
U.S. Pat. Nos. 4,056,593 and 4,959,265 disclose an early method of extruding polymeric webs with upstanding stems, known as stemmed webs. The stemmed web is formed of a single material. In the hook structure of U.S. Pat. No. 5,077,870, a single component thermoplastic resin is extruded into a tool which has an array of cavities which, upon separation, form an array of stems. The stems are then calendered to produce a broader head at the top of the stem. The shape, dimensions, and angularity of the flanges of the head and the number of stems per area determine the ease of capture and tenacity of hold of the loop onto the hook. The hook and stem material determines the flexibility of the, hook, the rigidity of the stem, and the friction of the hook to the loop. Some resins used in the hook structure are high modulus thermoplastics which provide suitable strength for supporting the hook structure but do not provide adequate flexural strength to prevent the hooks from fracturing or breaking during release of the loop. Also, the hook does not provide low friction for the movement of the loop from the top to the underside of the hook.
U.S. Pat. No. 5,393,475 discloses a method of making a stemmed web with stems on both sides using two different materials. This patent discloses extruding two different materials to form two base portions and forming hooks by allowing the material to fill cavities on two rollers between which the material passes. After, this, the base portions are pressed between the two rollers to laminate or stick them together. In one embodiment a third layer with a chemical affinity for the first two layers is used. This process would necessarily cool the two streams before lamination.
Hooks also can be made using profile extrusion, forming a long rib on the web. The rib is then laterally sliced and then bent or stretched to form a plurality of stems. Heads can be formed on the stems either before or after slicing.
There is a need for stemmed webs, such as mechanical fasteners, having a wider variety of properties to meet more varied applications.